The present invention relates to generally a magnetic chuck of the type in which when magnetic circuits are switched, a workpiece can be tightly held on the workpiece holding surface or can be released therefrom, and more particularly to a permanent magnetic chuck which can be turned on and off by upper (stationary) permanent magnets mounted on a workpiece holding plate and lower permanent magnets of a sliding member.
Referring first to FIG. 1, a prior art magnetic chuck will be described. FIG. 1 is a fragmentary longitudinal sectional view of a prior art magnetic chuck. FIG. 1(a) shows the OFF state (the de-energized state) while FIG. 1(b) shows the ON state (for tightly holding a workpiece on the workpiece holding surface). A base plate 1 made of a nonmagnetic material has a vertical wall 2 made of a nonmagnetic material. Magnetic pole pieces 3 made of a soft iron and permanent magnets 4 are alternately laminated and magnetic isolators 5 are interposed between the pole piece 3 and the permanent magnet 4. Thus, a workpiece holding plate is provided. Magnetic pole pieces 6 made of a soft iron and permanent magnets 7 are alternately disposed within a box defined by the base plate 1, the wall 2 and the workpiece in such a way that the same poles of the adjacent permanent magnets 7 are opposed to each other. The pole pieces 6 and the permanent magnets 7 constitute a sliding magnet member.
Next the mode of operation of the prior art magnetic chuck with the above-described construction will be described. When the sliding magnetic member is located at the position shown in FIG. 1(a), a magnetic circuit F.sub.1 does not appear at the top surface of the workpiece holding plate. As a result, the magnetic chuck is in the OFF state so that a workpiece is released. When the magnetic member is located at the position as shown in FIG. 1(b), the magnetic circuit F.sub.2 appears at the top surface of the workpiece holding plate. As a result the magnetic chuck is turned on so that a workpiece is tightly held in position.
In the prior art magnetic chuck as shown in FIG. 1, the magnetic chuck is turned on or off by sliding the lower magnets relative to the upper magnets (the workpiece holding plate) so that an excellent residual magnetism characteristic can be obtained. However, the directions of the magnetic poles of the sliding permanent magnets are in parallel with the workpiece holding plate (in the horizontal direction). As a result, the position of the workpiece holding plate (from the bottom surface of the base plate 1) must be raised. In addition, when the magnetic pole pieces 3 made of a magnetic material are moved away from the magnetic pole pieces 6 when the magnetic chuck is turned on or off, a heavy load is produced due to the magnetostriction phenomenon in which the magnetic paths between the pole pieces 3 and 6 become narrower according to the movement of the magnetic pole pieces 3 away from the magnetic pole pieces 6, increasing magnetic resistance in said magnetic paths. As a result, the prior art magnetic chuck has a disadvantage that a great force is required to turn on or off the magnetic chuck and consequently a sliding mechanism which can withstand such great force must be provided.